COUPLED HELICES 



153 



K = 





F{ya) 



In Fig. 2.9, Fiya), for various I'atios of inner to outer radius, is plotted 

 for both the transverse and longitudinal modes together with the value 

 of F{ya) for the single helix {b/a = co). We see that the longitudinal 

 mode has a higher impedance with cross wound coupled helices than 

 does a single helix. We call attention here to the fact that this is the 

 same phenomenon which is encountered in the contrawound helix^, where 

 the structure consists of two oppositely wound helices of the same radius. 

 As defined here, the transverse mode has a lower impedance than the 

 single helix. This, however, is not the most significant comparison; for 

 it is the transverse field midway between helices which is of interest in 

 the transverse mode. The factor relating the impedance in terms of the 

 transverse field between helices to the longitudinal field cni the axis is 

 Er (f)/Ei(0), where f is the radius at which the longitudinal component 

 of the electric field E^ , is zero for the transverse mode. This factor, 

 plotted in Fig. 2.10 as a function of /3oa cot \l/r , shows that the impedance 

 in. terms of the transverse field at f is interestingly high. 



1.00 



0.72 



1.6 2.0 2.4 



/3o a COT Ifi 



4.0 



Fig. 2.8 — The values of cot ^^./cot \pi required for complete power transfer 

 plotted as a function of /3tia cot \pi for several values of b/a. For comparison, the 

 value of cot ^2/cot \//i required for equal velocities on inner and outer helices is also 

 shown. 



